Age-hardenable austenitic stainless steel



Sept. 5,1967 E. J. DULIS AGE-HARDENABLE AUSTENITIC STAINLESS STEEL Filed March 29, 1965 v 6 5 4 3 2 Q Q 2: 2.5.3 v

Silicon (Percent) Chromium (Percent) INVENTOR. EDWARD J. DUL/S Agenf United States Patent 3,340,046 AGE-HARDENABLE AUSTENITIC STAINLESS STEEL Edward John Dulis, Mount Lebanon, Pa., assignor to Crucible Steel Company of America, Pittsburgh, Pa., a corporation of New Jersey Filed Mar. 29, 1965, Ser. No. 446,469 8 Claims. (Cl. 75-126) This application is a continuation-in-part of application Ser. No. 427,516, filed Jan. 22, 1965, now abandoned.

This invention pertains to age-hardenable stainless steel, and in particular, to such steel which exhibits improved strength and oxidation resistance in the age-hardened condition and excellent service life in contact with hot, sulfur-containing materials.

Age-hardened austenitic stainless steel is known, having been disclosed in, for example, Payson US. Patent No. 2,706,696, Payson et al. US. Patent No. 2,686,116, and Canadian Patent No. 632,186. Such steel is of use wherever non-magnetic steel of good elevated-temperature creep strength is required. I have now discovered, however, that for uses involving exposure of the steel to atmospheres containing sulfur and its compounds, it is particularly essential that the steel contain essentially no nickel. Nickel forms with sulfur a relatively low-melting compound, nickel sulfide, and I have discovered that in uses that involve exposure of the steel to a hot (1400 to 2000 F.) sulfur-containing atmosphere, far greater service life is obtained with .a steel that is essentially nickel-free. By the term nickel-free, I mean to include only those steels that do not have a purposeful addition of nickel made to them, either by the use of nickel-bearing scrap or by the addition of nickel as an alloying element. Such nickel-free steel would, of course, contain under 0.75%, and more usually under 0.30%, of nickel.

The Metals Handbook, 8th edition, pp. 576-579, shows that in certain applications in the chemical industry, it is known to use striaght-chromium steels such as Type 446 (27 Cr) and Type 430 (17 Cr), but such steels, being essentially ferritic, are far inferior in elevated-temperature strength to the steels of the present invention.

Sulfur-resistant age-hardenable austenitic stainless steel will find use, for example, in diesel engine valves, in equipment for making carbon electrodes for use in the electric furnace (which equipment is during use exposed to a mixtureof graphite and sulfur-bearing tar or pitch), in apparatus used in that part of the petroleum-refining industry which is concerned with the refining of sulfurbearing crude oils, in the roasting of sulfide-type ores, and in numerous other places in the chemical process industries and elsewhere wherein metal is exposed to the action of a hot sulfur-containing material in the gaseous, liquid, or solid state. As an example of the improved performance obtainable with the use of steels of the above invention, diesel engine valves made of steel of the instant invention exhibit a service life superior to that of the best steel hitherto used for such purpose.

In age-hardenable austenitic stainless steels, it has hitherto been considered desirable that the steels exhibit a microstructure that is at all times completely austenitic,

"ice

except for the precipitation, upon age hardening, of finely dispersed carbides. That is to say, chromium-manganesecarbon-nitrogen steels are known which exhibit, upon rapid cooling from a sufliciently high solution temperature (e.g., about 2200 F.), a microstructure composed entirely of austenite, the steels being in such condition relatively soft (under 30 C Rockwell), and which, upon aging for about 16 hours at about 1350 F., exhibit a microstructure composed of austenite with a fine carbide phase dispersed throughout the steel, especially within the grains, rather than at the grain boundaries. The steel exhibits considerable hardness in such condition, e.g., over 38 C Rockwell. Such steel is free of both the ferrite phase andthe sigma phase. Ferrite tends to form whenever ferrite-promoting elements, e.g., chromium, molybdenum, vanadium, tungsten, columbium, or tantalum, are present in larger amounts in comparison with the amounts present of austenite-promoting elements, e.g., nickel, manganese, carbon, and nitrogen. Sigma tends to form whenever the steels total content of certain elements (chromium, nickel, silicon, etc.) becomes too high. It has hitherto been considered desirable to avoid the formation of both ferrite and sigma phase. Ferrite has a relatively low strength at moderately elevated temperature (1200 to 1600 F.), and the sigma phase similarly affects the properties detrimentally, especially when it is permitted to occur in the steel in the form of relatively massive intergranular particles, as it sometimes does in Type 310 steel (25% Cr, 20% Ni, 1.50% max. Si, 0.25% max. C, 2.00% max. Mn, balance essentially iron).

Now, in accordance with the present invention, I have discovered that it is desirable to use, especially for applications involving exposure to hot sulfur-containing atmospheres or materials, a novel steel exhibiting a microstructure composed essentially of austenite, with a controlled intragranular precipitate of sigma-phase alloy being present in the steel in its age-hardened condition, the steel consisting essentially of the following elements in the amounts indicated, in percent by weight, approximately:

Except for unavoidable impurities and other elements in minor amounts not detrimentally aifecting the properties of 'the steel.

It is also considered essential that the composition of the steel not only fall within the ranges specified above but also be balanced in its respective contents of carbon and nitrogen, on the one hand, and chromium and silicon, on the other hand, so that the steel contains sufficient carbon and nitrogen to ensure that no ferrite or martensite is formed upon quenching or cooling from the sglption-treating temperature. To that end, it is essential that the composition of the steel substantially satisfy the equation C+N 0.078(Cr+1.4 Si12.5)

where C=carbon content, weight percent N=nitrogen content, weight percent Cr=chromium content, weight percent Si=silicon content, weight percent Phosphorus to 0.30 Sulfur 0 to 0.40 Vanadium 0 to 1.0 Molybdenum 0 to 2.0 Tungsten 0 to 2.5 Columbium 0 to 1.5 Tantalum 0 to 3.0

A phosphorus addition detracts from the forgeability of the steel but lowers somewhat the solution temperature required in heat treatment and promotes at elast slightly the strength of the steel. A sulfur addition improves machinability. Strength may be improved by adding one or more of the carbide-forming elements vanadium, molybdenum, tungsten, columbium, and tantalum, the total amount of these elements preferably being under 7 weight percent. Whenever any of the above-mentioned carbideforming elements is added, allowance must be made for appropriate adjustment of the required amount of carbon plus nitrogen. That is to say, the contents in weight percent of the five carbide-forming elements mentioned must be multiplied by appropriate coefficients, indicated below, and added to the quantity within the parentheses on the right-hand side of the equation given above. The coefficients to be used are as follows: 2.3 for vanadium, 1.4 for molybdenum, 0.63 for tungsten, 2.8 for columbium, and 1.4 for tantalum.

In applications that involve the use of the steel at high temperature in contact with oxygen-containing atmospheres, it is considered desirable that the steel not contain any molybdenum or vanadium, inasmuch as these elements are known to form volatile or molten oxides.

In place of or in addition to sulfur, the steel may con tain appropriate amounts of other machinability-promoting elements such as tellurium, bismuth, lead, or selenium.

It may prove advantageous to add to the steel a small amount of titanium or another element capable of influencing the morphology of the inclusions of sulfides or other machinability-improving inclusion in the steel. Use of amounts of titanium greater than about 0.5% are not recommended, however, as such larger amounts of titanium tend to react with the nitrogen in the steel.

Small additions of boron, alone or with zirconium, may in certain cases improve the hot formability and elevated-temperature strength of the steel.

More particularly, I prefer to use steel within the somewhat narrower ranges specified below, the numerals also having reference to composition in percentage by weight:

Carbon 0.40 to 0.60 Nitrogen 0.40 to 0.60 Chromium 18.0 to 21.0 Manganese 10.5 to 14.0 Nickel 0 to 0.35 Silicon 2.00 to 3.00 Iron 1 Balance Except for unavoidable impurities and other elements in niinoii amounts not detrimentally affecting the properties of t e s eel.

It is especially preferable to utilize nickel-free Cr-Mn-C-N austenitic steel having a silicon content related to the chromium content of the steel as indicated in the following tabulation:

Chromium Content,

Silicon Content, Weight Percent Weight Percent 18. 0 About 3.0 19.0 2.6 to 3.0 20.0 2.3 to 3.0 21. 0 2.0 to 3.0 22.0 2.0 to 3.0

Such steel will exhibit particularly good resistance to oxidation in air or another oxygen-containing atmosphere at an elevated temperature such as 15002200 F., as evidenced by a weight loss, after six 16-hour cycles of exposure in air at 2150 F. (1177 C.) of 0.5 gram per square inch or less. This value compares quite favorably with the figure of about 6 or 7 grams per square inch under the same conditions observed when testing another steel (2lCr4Ni-9Mn0.25Si-0.4N-0.5C) now used for diesel engine valves.

Steel of the present invention is, like the other agehardenable austenitic stainless steel of prior art, preferably heat-treated by first heating to a solution-treatment temperature of 2100 to 2300 F. (e.g., 2150 F.), then rapidly cooling by quenching in oil or water, and then finally aging to a desired hardness by heating of a substantial period such as about 10 to 100 hours at a temperature of about 1600 to 1200 F. The steels of the invention have imparted to them by such heat treatment good strength and hardness, such as a 100-hour creep-rupture strength at 1200 F. of 41,000 p.s.i., a room-temperature hardness, as heat-treated, of Rockwell C 40 or greater.

The properties and usefulness of steels of the present invention Will be apparent from data presented below.

In Table I there are presented results of elevatedtemperature tensile properties that are typical of steels of the present invention. As a specific preferred embodiment of such steels, the following composition is set forth: 21% chromium, 0.2% nickel, 12% manganese, 0.50% carbon, 2.8% silicon, 0.40% nitrogen, and the remainder iron expect for unavoidable impurities in minor amounts not detrimentally affecting the properties.

TABLE I.-ELEVATED-TEMPERATURE TENSILE PROPERTIES Hardness Test 0.2% Yield Tensile Elong. in 1.4 Red. 0! Condition (Re) Temp. Strength Strength in. (percent) Area F.) (1,000 p.s.1.) (1,000 p.s.i.) (percent) Hot rolled 38 1, 200 55 31 49 Do 38 1,350 44 61 32 55 1,900 F., 1 hr., WQ 32 1, 200 37 79 38 49 1,900 F., 1 hr., WQ 32 1, 350 34 61 46 50 2,150 F., 1 hr., W 25 1,200 32 78 39 42 2,150 F., 1 hr., WQ, 25 1,350 31 62 25 30 2,150 E, 1 hr., WQ 26 1, 500 30 43 22 28 2,150 F., 1 hr., W 26 1, 650 22 32 41-50 38-53 2,150 E, 1 hr., WQ. 26 1, 800 15 21 57-84 43-66 Tables II and III present creep-rupture test data that are typical of steel of the present invention.

TABLE II.CREEP-RUPTURE TESTS Data substantiating this effect are presented in the following Tables VI and VII. Table VI presents the chemi- Reduction of Area (percent) Stress Rupture (1,000 p.s.i.)

Elongation Life (hr.)

- Test Temp.

( F.) (percent) Minimum Creep Rate (percent/hr.)

w Htt-H- Hbwe w aoassmmaewoaeeee TABLE III.CREEP-RUPTURE STRENGTH Stress (1,000 'p.s.i.) for Rupture incal compositions of certain steels investigated and Table VII presents the oxidation-test results.

TABLE VI.COMIOSITION OF STEELS Test Temp. F.)

10 hr. 100 hr. 500 hr. 1,000 hr.

A comparison of the foregoing data with that presented in the Metals Handbook, 8th edition, 1961, pp. 627-629, reveals that steels of the present invention exhibit a creep strength very similar to that of the known valve steel 21-4-N, indicated by data in the handbook to be. one of the most creep-resistant of the known iron-base valve materials.

TABLE IV.HOT-HARD NESS TESTS It has been observed that nickel in austenitic stainless steel promotes oxidation resistance, and that steels with little or no nickel are interior as respects this property. The above-mentioned Payson US. Patent No. 2,706,696 contains a broad suggestion that additions of silicon in amounts of 1.5 to 3.0% improve oxidation resistance. I have discovered, however, that an improvement in oxidation resistance of greater magnitude than would be predictable from the above-mentioned teaching of Patent No. 2,706,696 can be obtained by careful control of the contents of silicon and chromium, using silicon conten as given in the following Table V.

TABLE V Chromium Content, Silicon Content, W eight percent Weight Percent 18 About 3.0

. Composition (Weight percent) Steel N0.

0 Mn s1 Cr N Ni Steels 64-13 8 to 64-151 are varlous nlckel-free CrMnCN steels with various added amountsof silicon, and steel 63-130 is the above-mentioned steel 21-4-N now used for engine valves.

TABLE VIL-COMPOSITIO IFEgQISSSTEELS FOR OXIDATION Compositional Six 16 Hr. Variables (percent) Cycles at 2,150 F. Steel No.

Cr Si N Weight Loss,

g./in.

An appreciation of the criticality of the inter-relation between chromium content and scilicon content can be obtained from an examination of FIGURES 1 and 2, in which the above data is plotted.

To show the usefullness of the steel of the present invention as material for diesel engine valves, and inferentially, for other purposes involving exposure to hot media containing sulfur and oxygen, attention is directed to the following Table VIII. Diesel engine valves were made from steel of the instant invention and from a number of other known materials, and service-life tests were conducted.

TABLE VIIL-DIESEL ENGINE TEST RESULTS Nominal Composition, Weight Percent Material Relative Life Cr Ni Co Mn Si Mo W Fe Ti Al 30 21. 3 20.0 20.0 1. 5 0. 0. 50 Inconel X 18 15. 5 Bal. 0.7 0.6 0.05 0.3

The foregoing data show that steel of this invention is significantly superior to existing diesel valve steels and compares favorably in service life with the high-cost highly alloyed superalloy materials.

While I have shown and described herein certain embodiments of the present invention, I intend to cover as Well any change or modification therein which may be made without departing from the spirit and scope of the invention.

I claim:

1. Age-hardenable austenitic stainless steel consisting essentially of, in weight percent,

Carbon 0.35 to 0.75 Nitrogen 0.35 to 0.75 Chromium 18.0 to 22.00 Manganese 10.5 to 14.0 Nickel O to 0.75 Silicon 2.00 to 3.00 Phosphorus 0 to 0.30 Sulfur 0 to 0.40 Vanadium 0 to 1.0 Molybdenum 0 to 2.0 Tungsten 0 to 2.5 Columbium 0 to 1.5 Tantalum 0 to 3.0 Iron Balance said steel substantially satisfying the equation where in the above equation the chemical symbol for a given element signifies the content of that element in the steel in weight percent, the total content of the elements vanadium, molybdenum, tungsten, columbium and tantalu-m being under 7 weight percent.

2. A diesel engine valve made of steel as defined in claim 1.

3. Age-hardenable austenitic stainless steel consisting essentiall of, in Weight percent,

Carbon 0.35 to 0.75 Nitrogen 0.35 to 0.75 Chromium 18.0 to 22.0 Manganese 10.5 to 14.0 Nickel 0 to 0.75 Silicon 2.00 to 3.00 Iron Balance said steel substantially satisfying the equation C+N 0.078(Cr+1.4Si- 12.5)

claim 3.

5. Age-hardenable austenitic stainless steel consisting essentially of, in weight percent,

Carbon 0.35 to 0.75 Nitrogen 0.40 to 0.60 Chromium 18.0 to 20.0 Manganese 10.5 to 14.0 Nickel 0 to 0.35 Silicon 2.00 to 3.00 Iron Balance said steel substantially satisfying the equation C+N 0.078(Cr+1.4Si-12.5)

where C=carbon content, weight percent N=nitrogen content, weight percent Cr=chromium content, weight percent Si=silicon content, weight percent 6. A diesel engine valve made of steel as defined in claim 5.

7. Steel as defined in claim 5, characterized in that said steel has a silicon content related to the chromium content of the steel substantially as follows Chromium Content, Silicon Content, Weight Percent Weight Percent 18. 0 About 3.0 19.0 2.6 to 3.0 20.0 2.3 to 3.0 21. 0 2.0 to 3.0

8. A diesel engine valve made of steel as defined in claim 7.

References Cited UNITED STATES PATENTS DAVID L. RECK, Primary Examiner.

P. WEINSTEIN, Assistant Examiner. 

1. AGE-HARDENABLE AUSTENITIC STAINLESS STEEL CONSISTING ESSENTIALLY OF, IN WEIGHT PERCENT, CARBON 0.35 TO 0.75 NITROGEN 0.35 TO 0.75 CHROMIUM 18.0 TO 22.00 MANGANESE 10.5 TO 14.0 NICKEL 0 TO 0.75 SILICON 2.00 TO 3.00 PHOSPHORUS 0 TO 0.30 SULFUR 0 TO 0.40 VANADIUM 0 TO 1.0 MOLYBDENUM 0 TO 2.0 TUNGSTEN 0 TO 2.5 COLUMBIUM 0 TO 1.5 TANTALUM 0 TO 3.0 IRON BALANCE SAID STEEL SUBSTANTIALLY SATISFYING THE EQUATION 